Modern wind energy installations are designed for variable rotation speeds and, for this purpose, are provided with a converter. Normally, they have a doubly-fed asynchronous generator, whose stator is permanently connected to the grid, and the rotor is connected to the grid via the converter. This allows the generator to be operated not only at a rotation-speed which corresponds to the grid frequency but also at a lower rotation-speed than that corresponding to the grid frequency (or the synchronous rotation-speed that is governed by it), or operation at a higher rotation-speed than that corresponding to the synchronous rotation-speed. The respective discrepancy between the actual rotation-speed and the synchronous rotation-speed, which is governed by the grid frequency, is referred to as slip. Slip values of ±30%, or in some cases even more, are normal for modern wind energy installations. The wind energy installation can therefore be operated over a wide rotation-speed range.
The synchronous rotation-speed, which is critical for determining the slip, is not constant during practical operation on the actual grid, but is subject to the same fluctuations as the grid frequency. For example, when an overfrequency occurs in the grid, then the synchronous rotation-speed is correspondingly shifted upwards, and vice versa. If the slip values in consequence become too high or too low, then there is a risk of overloading and of damage to components of the wind energy installation. Various remedial areas to avoid this are known from the prior art.
A first measure is to base the design of the components of the wind energy installation on the respectively worst extreme case, that is to say to take account of the maximum permissible grid frequency discrepancy. In the end, this leads to planned overengineering of the components, and is therefore correspondingly expensive, in terms of production costs. Furthermore, when the characteristics are matched to the worst case, this leads to sub-optimum operation at the nominal frequency, resulting in yield losses. In addition, this design based on the extreme case is still completely inadequate and, furthermore, the steady-state rotation-speed range must be matched to the grid frequency in order to prevent overloading in the event of excessive rotor slip, in particular in the event of a relatively heavy load, when an actual underfrequency occurs.
US 2007/069522 A1 discloses a different approach for the adaptation of the characteristics. The grid frequency is measured in order to determine whether an overfrequency or underfrequency situation exists, in order to shift the rotation-speed/torque characteristics toward the synchronous point, as a function of this. This reduces the slip that actually occurs. The entire characteristic is therefore adapted as a function of the actual grid frequency. This has the disadvantage that this adaptation acts over the entire operating range, which can likewise result in yield losses as a result of unnecessary reduction, as already described above for the static design based on the extreme case. Furthermore, this results only in inadequate protection for certain operating states, for example for the combination of overfrequency and high load.
Adequate protection cannot be ensured in particular for this critical operating situation.